1. Field of the Invention
The invention is directed to a transfer valve for use in a system wherein a fluid must be diverted from one work load to a second work load while maintaining uninterrupted operation of the system. An example of such a system is a filter strainer circuit or a heat exchanger circuit wherein one unit must be cleaned or repaired without interrupting the operation of the overall system. Generally speaking, any system which includes a standby unit which must be intermittently integrated into the system to maintain continuous operation of the system will advantageously incorporate the transfer valve of the invention.
2. Description of the Prior Art
Transfer valves presently in use are generally double plug valves or ball valves with two downstream check valves to isolate return flow. Plug valves tend to freeze after remaining in one position for a long period of time, and in an attempt to minimize this problem, a lubricant is used on a continuous basis. Additionally, a plug valve requires a substantial turning torque on the valve actuator to change the position of the plug in the valve casing due to the extremely large frictional drag on the plug surface when it is moved relative to the valve casing. Furthermore, a plug valve operates relatively slowly so that there is a substantial pressure drop in the system when the plug is adjusted. The design and the large amount of torque required to operate a plug valve limit the practical size of such a valve especially in a double three way arrangement to approximately a six inch valve which is not large enough for many commercial installations. A ball valve will have the same basic deficiencies as a plug valve and, therefore, is not a practical substitute for installations requiring large valves.